The present invention relates to biaxially-oriented films and containers provided with high resistance to gas permeability, and to the polymeric materials used to prepare said containers and films.
Aromatic polyester resins are used in ever-increasing amounts in the production of beverage containers and films.
The barrier properties of aromatic polyester resins are rather limited. In the case of containers for carbonated beverages prepared from said resins, the possibility to preserve the beverages for a sufficiently long time is uncertain.
Polyamide resins have remarkable mechanical properties, but they have the drawback that they have a high moisture absorption which reduces their properties.
Polyamide resins are normally used mixed with aromatic polyester resins in order to improve the mechanical characteristics of the latter. The presence of polyester resin in the mix reduces the moisture-absorbing tendency of polyamide resins.
Mixing the resins, however, is difficult because of their poor compatibility in the melted state.
In order to obtain better mechanical properties and to avoid peeling in products, it has been suggested to mix the resins in the extruder in the presence of a dianhydride of a tetracarboxylic acid (JP 1-272660 Kokai).
Pyromellitic anhydride is the preferred compound.
The mechanical properties of the resulting mixtures can be improved further by subjecting the mixtures to a solid-state polycondensation treatment (WO 94/09069).
Among polyamides, the most commonly used polyamides, such as nylon 6 and 66, have slightly better gas barrier properties than polyester resins such as polyethylene terephthalate (PET) and copolyethylene terephthalates such as polyethylene terephthalate (PET) and copolyethylene terephthalates containing small proportions of units derived from isophthalic acid.
However, a polyamide obtained from m-xylylene diamine and adipic acid (poly-m-xylylene adipamide, poly MXD-6) is known as having considerable gas barrier properties (in relation to oxygen and carbon dioxide) which are distinctly better than those of polyethylene terephthalate.
This polyamide is used in mixture (obtained in an extruder) with PET or COPET in order to improve their barrier properties.
The oxygen permeability of a 1.5-liter PET bottle (produced by injection blow-molding) is reduced by approximately 50% when the bottle is obtained from a mixture which contains 16% by weight of polyamide and by approximately 20% when it contains 7% polyamide by weight.
The barrier properties of the 16% polyamide mixture are similar to those of a bottle formed of a two-layer film, one layer being PET and the other one being polyamide.
It has now been found unexpectedly that it is possible to remarkably improve the barrier properties of biaxially-oriented films and containers which can be obtained from polyester resins used in a mixture with a polyamide such as poly MXD-6 if the polyester resin is first mixed in the melted state with a dianhydride of a tetracarboxylic acid and the resulting mixture is further mixed, again in the melted state, with a polyamide such as poly MXD-6, working under temperature and shear forces conditions and with viscosities of the melted polymeric components such as to render the polymeric components of the mixture compatible from the rheological viewpoint. The dianhydride is mixed with the polyester resin in an amount from 0.01 to 3% by weight.
Pyromellitic dianhydride is preferred. Other dianhydrides that can be used are dianhydrides of 1,2,3,4-cyclobutane tetracarboxylic acid, 3,3xe2x80x2:4,4xe2x80x2-benzophenone tetracarboxylic acid, 2,2 bis-(2,4-dicarboxyphenyl)ether. The dianhydrides of aromatic tetracarboxylic acids are preferred.
Preferably, the polyester resin is a copolyethylene terephthalate in which up to 25%, preferably 1 to 15%, of the units derived from terephthalic acid are substituted by units or sequences derived from isophthalic acid or from mixtures thereof with naphthalene dicarboxylic acids.
The polyamide is preferably poly MXD-6. Other polyamides that can be used are those obtainable from an aliphatic dicarboxylic acid other than adipic acid containing 6 to 22 atoms of carbon and an arilene diamine, preferably m-xylylene diamine.
Said dicarboxylic acid is preferably suberic, sebacic and dodecanoic acid.
Polyamides with crystallization rates similar to those of polyester are preferably used.
The numeral molecular weight of the starting polyamide is generally between 8000 and 50000.
The terminal-NH2 groups of the starting polyamide can be reacted with epoxy compounds in order to increase its dispersion in the polyester matrix. Epikote from the Shell Italia is an example of usable epoxi compounds.
Preferably, the polyamide is used in an amount equal to 5 to 30% by weight on the mixture. Larger amounts can be used depending on the final properties of the mixture.
It is in fact possible, and it is another aspect of the invention, to prepare, operating according to the method of the invention, mixtures having a polyamide content of more than 50% by weight and up to 80-90% by weight which can be used as master batch.
A preferred mixing process consists in extruding the polyester resin with the addition of the dianhydride and in subsequently extruding the mixture with the addition of the polyamide.
It is possible to work in a single stage by premixing in the extruder the polyester and dianhydride first and then adding the polyamide in the extruder.
In order to achieve good compatibilization among the polymeric components, their viscosities in the melted state has to be very similar.
Preferably, the ratio between the viscosity of the polyester and the viscosity of the polyamide is between 0.2:1 and 4:1.
Operating under the above conditions it is possible to obtain compositions wherein the polyamide is dispersed in the polyester matrix with domains having size of less than 1 micron, preferably in the range from 0.2 to 0.4 micron. The microstructure was obtained by scanning electron microscope (S.E.M.) of the fracture surface of injection molded small bars treated with formic acid to extract the polyamide. The bars obtained according to example 1 have a microstructure wherein the dispersed phase has a size of less than 0.5 micron. The size of the domains in the bars obtained from example 1 but without using PMDA are higher than 1.5 micron as average. The microstructure according to the invention is characteristic in particular of the blends of poly MXD-6 with PET or copolyethylene thereftalates containing up to 25% of units deriving from isophtalic acid.
It is surprising that when working under the mixing conditions according to the invention it is possible to obtain materials having gas barrier properties (relative to O2 and CO2) which are far higher than those obtainable by mixing the polyester resin and the polyamide in the absence of the dianhydride of tetracarboxylic acid or by mixing the three components of the mixture simultaneously or by first mixing the polyamide with the dianhydride and then adding the polyester resin.
Oxygen permeability in 1.5-liter bottles with an average thickness of 225 microns can be reduced of 4 or more times and CO2 permeability can be reduced by 2 or more times as a function of the content of the polyamide (pMXD-6).
The polyester resin is obtained by polycondensation (according to known methods) of terephthalic acid or lower diesters thereof with a diol with 2-12 carbon atoms, such as for example ethylene glycol, 1,4-butanediol and 1,4-cyclohexane dimethylol. The copolyethylene terephthalate is, as mentioned, the preferred resin for preparing containers.
The polyethylene terephthalate homopolymer can be conveniently used in the preparation of biaxially-oriented films.
The starting polyester resin used in the preparation of the compositions according to the invention has an intrinsic viscosity of 0.3 to 0.8 dl/g. The initial viscosity can be increased by subjecting the resin, premixed with the dianhydride of the tetracarboxylic acid or the mixture containing the polyester resin, the polyamide and the dianhydride, to a solid-state polycondensation treatment at temperatures between approximately 150 and 230xc2x0 C. for a time and at temperature conditions sufficient to increase by at least 0.1 units the viscosity of the polyester resin.
Since the polyester resin is difficult to separate by solvent extraction in the mixture, the increase in intrinsic viscosity of the polyester resin in the mixture is considered similar to the increase in the polyester resin when it is treated alone in the same temperature and duration conditions to which the mixture has been subjected.
The solid-state polycondensation treatment, in addition to leading to an increase in the intrinsic viscosity of the polyester resin, allows to improve the mechanical properties of the compositions, particularly impact resistance.
The treatment applied to the polyester resin added of the dianhydride of the tetracarboxylic acid allows to bring the viscosity of the resin in the melted state to values which are similar to those of the polyamide resin, particularly when the initial polyester resin has relatively low intrinsic viscosity values.
The mixing of the polyester with the dianhydride and then with the polyamide is performed in a single- or twin-screw extruder. Contrarotating and intermeshing twin-screw extruders are preferred, using residence times of generally less than 180 seconds and working at temperatures above the melting temperatures of the polymeric components, generally between 200xc2x0 and 300xc2x0 C.
The biaxially oriented films and containers are prepared according to known methods. For example, bottles for beverages are prepared by injection-stretch blow molding; biaxially oriented films are prepared with the double-bubble method or by cast-extrusion followed by biaxial stretching.
The material according to the invention can also be used to prepare multilayer films comprising, as a core layer, a biaxially oriented film obtained from the material according to the invention.
Another application of the blends according to the invention, wherein the polyester resin is a copolyethylene therephthalate with 10% or more of units from isophthalic acid resides in the preparation by free blowing of preforms of high capacity pouches (51. or more) suitable for being filled with liquids such as soft drinks or others.
The pouches have high clarity and good mechanical properties. They can be easily folded without breakage or stress withening problems.
The intrinsic viscosity is measured in solutions of 0.5 g of resin in 100 ml of 60/40 solution by weight of phenol and tetrachloroethane, at 25xc2x0 C. according to ASTM standard D 4603-86.
The measurement of the melt viscosity is performed with a Goettfert rheometer equipped with a 2-mm capillary tube, working at 280xc2x0 C. with a shear rate of 100 secxe2x88x921.